Final answer:
Diploid dominant life cycles have a multicellular diploid stage, as seen in humans. Haploid dominant cycles have a multicellular haploid stage, typical for fungi. Alternation of generations, seen in plants, involves both haploid and diploid multicellular stages with gametophytes and sporophytes alternating.
Step-by-step explanation:
Distinguishing Different Life Cycles
In biology, understanding the different life cycles is crucial to studying the reproductive methods of different organisms. To distinguish among the life cycles of diploid dominant species, haploid dominant species, and species that exhibit an alternation of generations, one must look at the stages in which these organisms spend the majority of their lifecycle and where the mitotic cell reproduction occurs.
Diploid Dominant Cycle
In a diploid dominant cycle, the multicellular diploid stage is predominant, meaning that organisms such as humans spend most of their lifecycle in the diploid state, and the primary stage for mitotic reproduction is also diploid. Gametes are the only haploid cells, which serve to restore the diploid state after fertilization.
Haploid Dominant Cycle
Conversely, in a haploid dominant cycle like that of fungi and some algae, the multicellular haploid stage is dominant. These organisms spend most of their life cycle as haploids, with the zygote being the only diploid cell, which undergoes meiosis rapidly to revert to the haploid state.
Alternation of Generations
Species with an alternation of generations, a characteristic of most plants and some algae, undergo life cycles that alternate between haploid and diploid multicellular stages. The haploid gametophytes produce gametes, and following fertilization, the diploid zygote grows into a diploid sporophyte. Within the sporophyte, specialized cells undergo meiosis to produce haploid spores, which will then develop into gametophytes.